Method and architecture for providing telephony between data networks and PSTN

ABSTRACT

A method consistent with the present invention enables communicating telephonic data regarding a call over a data network. The method includes the steps of receiving data units from a first data network over redundant communication paths. Next, it is determined whether the received data units have an error. One of the received date units is then selected from one of the redundant communication paths determined not to have an error, and the selected data unit is forwarded to a second data network.

This application claims the benefit of Provisional application Ser. No.60/020,432, filed Jun. 25, 1996.

BACKGROUND OF THE INVENTION

A. Field of the Invention

This invention relates generally to methods for communicating data overa data network, and, more particularly, to a method that allowssignaling data to be communicated over the data network.

B. Description of the Related Art

Common Channel Signaling (CCS) provides a dedicated supervisory networkfor segregating signaling information from voice and data information ina telecommunications network. CCS was developed to meet the increaseddemands placed on the public telecommunications network by the growingmarket for voice, data, and information services. Previous signalingsystems sent call setup and routing information over the same trunkcircuit used for voice transmission. With CCS, a single out-of-bandchannel conveys signaling information relating to call setup, routing,and network management, among other things. Signaling System No. 7(SS7), an international protocol standard for CCS communications,creates a standard format for communicating signaling information in aCCS network (CCS7).

FIG. 1 diagrammatically illustrates a PSTN having a CCS7 network 110 anda voice network 130, each of which interfaces with a plurality ofservice switching points (SSPs) 120. SSPs 120 are located at a centraloffice to provide CCS7 trunk signaling and the capability to query adatabase to determine call routing. CCS7 network 110 includes STPs 112which route CCS7 messages between SSPs and STPs and control access tothe CCS7 network. In addition, each SSP 120 is connected to voicenetwork 130, such as a long-distance telephone network by voice trunks132.

The emergence of desktop computing, local area networks (LANs), and theInternet, brought the desire to carry CCS7 signaling data over datanetworks. Significant cost savings to communications providers could berealized if the CCS7 signaling data could be reliably transmitted overthe existing data networks. The savings would stem from not having toinstall and maintain separate Signaling Networks; which are known to beextremely expensive in a telephone network, due largely in part to theinherent complexity required to achieve the high degree of reliability.

Any approach using a data network to carry CCS7 signaling data must alsoconsider the reliability of the message transfer. In today's datacommunication networks, reliable messaging of signaling data isgenerally performed by either: 1) utilizing a rigorous protocolimplementation which corrects for lost messages; or 2) using fullyduplicated transmission paths to minimize the impact of a break in oneof the two transmission paths. In the most sensitive applications, suchas in today's telephone CCS7 Signaling Networks, these methods arecombined to obtain maximum reliability of message transfer. Thisapproach has a number of drawbacks. First, providing a duplicated andsegregated data network just for the signaling data is expensive.Second, the number of specialized CCS7 signaling data routers (i.e., theSTPs) increases the expense and the complexity of the system as well.

Within the computer industry, a different communication network hasemerged based on Local and Wide Area Networks (LANs & WANs). Thesenetworks achieve reliability not by duplicated physical communicationpaths, but by the network's ability to send messages based solely on adestination address and to have them arrive at the intended destinationthrough a number of diverse routes. However, the network itself does nottypically provide for guaranteed delivery of a particular message at theintended destination. The end points involved in a message exchangemust, therefore, implement a rigorous protocol to detect lost messagesand retransmit the detected lost messages. This is usually veryprocessor and memory intensive, and the recovery of lost messagesthrough retransmission is often slow-particularly when the network isgeographically diverse, such as the Internet.

Some data communication networks today can support limited voicecommunications across a data network. FIG. 2 illustrates a data network210, such as the Internet, connected to two telephony equipped personalcomputers (PC) 212 and 214. However, since data network 210 does notinterface with a PSTN in this system, any communication of signalingdata would be minimal and merely related to routing.

FIG. 3 illustrates a more advanced data network based system whichsupports voice communications. In FIG. 3, a telephone call connectionpath is formed for connecting a data network 310 to a PSTN 320 through atelephone gateway 350. A user of PC 312 on data network 310 may initiatea call by dialing the directory number (DN) of a telephone 322 on PSTN320. PC 312 sends the DN in a message over data network 310 to atranslation server 314, which uses the DN to determine the Internetprotocol (IP) address of a gateway 350 closest to phone 322. Translationserver 314 returns the IP address of gateway 350 to PC 312, which thensends the DN over data network 310 to phone gateway 350.

The system of FIG. 3, however, does not allow any signaling information(i.e., the calling party's name and number) to be delivered between adata network 310 and PSTN 320. In addition, since telephone 322 cannotoriginate and complete a call to a PC 312, businesses would stillrequire a traditional phone to receive calls from clients and customers.‘1-900’ calls dialed by PC 312 would be problematic since PSTN 320 wouldview telephone gateway 350 as the originator of the call and not PC 312.This occurs since phone gateway 350 effectively looks like a telephoneto PSTN 320 since it is connected to PSTN 320 by a link terminating on aline circuit at an end office switch of PSTN 320. Thus, this system isunable to communicate the full complement of signaling informationbetween a data network and a PSTN, prohibiting data network users fromtaking full advantage PSTN services.

Therefore, the above communication systems are not able to reliably andcost effectively transmit the full complement of signaling informationregarding a call between a data network and a PSTN. This poses a seriousbarrier to the merging or integration of computer based telephony andthe traditional telephone network PSTN.

SUMMARY OF THE INVENTION

Systems and methods consistent with the present invention provide auniversal, high speed, highly reliable gateway for enabling voice andsignaling communication between a data network and a PSTN.

To achieve these and other advantages, a method of communicatingtelephonic data regarding a call over a data network, comprising thesteps of: receiving data units from a first data network over redundantcommunication paths; determining whether the received data units have anerror; selecting one of the received data units from one of theredundant communication paths determined not to have an error; andforwarding the selected data unit to a second data network.

Both the foregoing general description and the following DetailedDescription are exemplary and are intended to provide furtherexplanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings provide a further understanding of theinvention and, together with the Detailed Description, explain theprinciples of the invention. In the drawings:

FIG. 1 illustrates a typical public switched telephone network (PSTN);

FIG. 2 illustrates a prior art data network system;

FIG. 3 illustrates a data network system having limited voicecommunications with a PSTN;

FIGS. 4A to 4C illustrate a signaling server based networks consistentwith the present invention;

FIG. 5 illustrates the different types of data interfaces of thesignaling server of FIG. 4;

FIG. 6 illustrates the use of virtual dual planes for creating redundantcommunication paths which interconnect a plurality of signaling servermodules located in the signaling server of FIG. 4;

FIG. 7 is a block diagram of a signaling server module consistent withone embodiment of the present invention;

FIG. 8 is a block diagram of a second signaling server module consistentwith another embodiment of the present invention;

FIG. 9 is a block diagram of a third signaling server module consistentwith yet another embodiment of the present invention; and

FIGS. 10 and 11 illustrate the receive and transmit cell steeringfunctions, respectively, of the signaling server module of FIG. 9.

DETAILED DESCRIPTION

Embodiments of systems consistent with the present invention will now bedescribed in detail. Wherever possible, the same reference numbers usedthroughout refer to the same or like parts.

Overview

Signaling servers consistent with this invention may be used tocommunicate signaling data in place of a CCS7 network or may be used tocommunicate voice data in place of a voice network, such as a typicallong-distance telephone network. In, addition, such signaling serversalso enable communication of voice and signaling data between a datanetwork and a public switched telephone network (PSTN). The term“signaling data” refers to the supervisory signals used in a CCS7network, and includes: call setup information, network managementinformation, and class service information.

To increase the reliability of the communicated data, the signalingserver receives and processes the voice and/or signaling data over aplurality of redundant communication paths. When transmitting data tothe network connected to the signaling server, the signaling serverchecks the data of each communication path for errors, and selects thedata from one of the paths which has no errors. The selected data isthen forwarded to the connected network. When receiving data from theconnected network, the signaling server replicates the data such thatthe same data is transmitted to the data network over each of theredundant communication paths.

Signaling Server Network Architecture

The signaling server may be used in a variety of network applications,as shown by FIGS. 4A to 4C. FIG. 4A illustrates a signaling server basednetwork consistent with the present invention in which the signalingserver replaces a portion of a CCS7 network. As shown in FIG. 4A, thesignaling server based network includes a signaling server 410, aplurality of SSPs 420 and a voice network 430, such as a long-distancetelephone network. SSPs 420 are each connected to signaling server 410and voice network 430 by signaling links 422 and voice trunks 424,respectively.

Signaling server 410 further includes a plurality of signaling servermodules 412 which connect to one or more signaling links 422. Signalingserver modules 412 further connect to a server data network 414 througha plurality of redundant communication paths 416. Server data network414 can then transfer signaling data from one signaling server module toanother. In this way, SSPs 420 can communicate signaling data betweeneach other though signaling server 410, effectively obviating the needfor a separate CCS7 network.

Signaling links 422 transmit and receive data according to the CCS7protocol. Signaling server modules 412 convert the CCS7 signalinginformation received over signaling links 422 into a message formatacceptable for server data network 414. Signaling server modules 412then forward the converted data to another signaling server module 412connected to server data network 414 through redundant communicationpaths 416.

In systems consistent with the present invention, server data network414 communicates data according to the asynchronous transfer mode (ATM)format. Accordingly, a signaling server 410 having an ATM server datanetwork 414 will be described below. However, signaling server 410 maybe used with server data networks 414 operating under othercommunication formats, such as the X.25 format or the TCP/IP addressingformat used by the Internet.

FIG. 4B illustrates a signaling server based network consistent with thepresent invention in which the signaling server bridges a data network,such as the Internet, to a PSTN. As shown in FIG. 4B, a signaling server410 allows a data network 440 to communicate voice and signaling data tovoice network 430 and CCS7 network 450 of a PSTN. Computers 442, eachinclude, for example, a modem and software for answering and initiatingtelephone calls, and are connected to data network 440. A high capacitylink 444, such as an Ethernet, connects data network 440 to signalingserver 410. Voice trunks 424 and signaling links 422, in turn, connectsignaling server 410 to voice network 430 and CCS7 network 450,respectively.

Signaling server 410 receives voice data over trunks 424 and signalingdata over links 422, and combines the received data for transmissionover high capacity link 444. Data network 440 receives the combined dataover link 444 and routes it to computer 442 that is addressed by thecombined data. In addition, signaling server 410 converts the receiveddata into a data format compatible with data network 440. Similarly,computer 442 can transmit voice and signaling data over data network 440and high capacity link 444 to signaling server 410. When signalingserver 410 receives voice and signaling data from data network 440, itwill separate the combined voice and signaling data for outputting overrespective voice bunks 424 and signaling links 422. The signaling serverbased network will thus allow all types of calls (i.e., local,long-distance, toll-free, or “1-900”) to be placed from a computer 442on data network 440 to a telephone connected to a SSP 420.

FIG. 4C illustrates a signaling server based network consistent with thepresent invention in which the signaling server replaces a portion ofthe voice network. As shown in FIG. 4C, the signaling server basednetwork includes a signaling server 410, a plurality of SSPs 420 and aCCS7 network 450. SSPs 420 are each connected to signaling server 410and CCS7 network 450 by voice trunks 424 and signaling links 422,respectively. In this way, SSPs 420 can communicate voice data betweeneach other though signaling server 410, effectively obviating the needfor the voice network.

Signaling Server

As described above, the signaling server can transfer either signalingdata, voice data, or both signaling and voice data. To this end, thesignaling server includes a multiple of data interfaces for serving in avariety of applications. FIG. 5 illustrates the two types of interfacesassociated with each signaling server. Type 1 interfaces include asignaling/control interface and a support interface, while Type 2interfaces include voice/circuit switch interfaces. The signaling servermay be configured to support any combination of these interfacesdepending upon the particular type of application.

In the signaling servers of FIG. 4, for example, Type 1 interfaces areused to configure the signaling server of FIG. 4A, while Type 2interfaces are used to configure the signaling server of FIG. 4C. Thesignaling server of FIG. 4B, however, is configured to interface withboth Type 1 and Type 2 data since it transfers both signaling and voicedata.

Type 1 interfaces communicate using protocol data units (PDUs),preferably using the ATM Adaption Layer 5 (AAL5) format. Theseinterfaces have a delay characteristic which allows the signaling serverto be tolerant of data delays and delay variations, and have a zerotolerance for corrupted data. Type 2 interfaces, on the other hand,communicate using a time division multiplexed (TDM) digital data stream,preferably using the ATM Adaption Layer 1 (AAL1) format. Type 2interfaces have a low tolerance for data delays and delay variations,and, as opposed to Type 1 interfaces, can tolerate some amount of errorsin the received data. The signaling server will then be configureddifferently for Type 2 interfaces than it will be for Type 1 interfaces.

As shown in FIG. 6, signaling server modules 412 communicate with oneanother through a virtual plane 615 located in server data network 414.Each signaling server module 412 transfers voice and/or signaling dataover a plurality of redundant communication paths 416 to a correspondingvirtual plane 615. Further, each virtual plane 615 terminates acorresponding redundant communication path 416 from each of thesignaling server modules 412.

Server data network 414 may be a back plane connectivity network withinsignaling server 410 itself. In this case, the virtual planes areessentially a physical connection between signaling server modules 412.Signaling server network 414 may also comprise a separate data network,such as the Internet. The virtual planes will then be established byforming connections, through the separate server data network, for eachredundant communication path 416.

Signaling Server Module

A signaling server module 412 will now be described in detail for use inthe above signaling server 410. As stated above, signaling server module412 may communicate either Type 1 data, Type 2 data, or both Type 1 andType 2 data. For the sake of brevity, a universal signaling servermodule will be described which can communicate both types of data.

As shown in FIG. 7, a signaling server module includes data interfaces710, data segmentation and reassembly (SAR) units 720, type 1 planeselection 730 and replication 740 units, re-transmission logic 750, type2 plane selection 760 and replication 770 units, type 1 interface 780,and type 2 interface 790.

Although the signaling server module performs standard interfacing anddata link level functions, it primarily performs a variety of redundantcommunication functions. These include data interface functions, datasegmentation and reassembly (SAR) functions, and planeselection/replication functions. To this end, the signaling servermodule includes a multiple of data interfaces 710 each of which receivesredundant voice and/or signaling data communicated over duplicatedcommunication paths 712 and 714 (same as paths 416 of FIGS. 4A to 4C).This helps ensure that an error-free data cell may be received by thesignaling server. Each path connects to a respective virtual plane.While the illustrated signaling server achieves redundancy throughduplication (i.e., only two communication paths 712 and 714, and onlytwo, ATM interfaces 710 are shown), higher order redundancy techniques,such as triplex, may be used.

When the signaling server module receives ATM data cells from the serverdata network, ATM interfaces 710 output the received ATM cells torespective ATM segmentation and reassembly (SAR) units 720 overcorresponding redundant communication paths. Each ATM SAR unit 720outputs Type 1 interface data on a PDU bus and outputs Type 2 interfacedata on a TDM bus.

Type 1 plane selection unit 730 receives over the PDU bus the PDUs fromeach ATM SAR unit 720, and selects the first PDU having no errors.Selection unit 730 preferably determines whether a PDU contains errorsbased on a cyclic redundancy check (CRC). As known in the art, a CRCinvolves running an equation on the data stream prior to transmission,and placing the result of the equation in a check sum field of the PDU(referred to as a CRC code). A receiver then runs the same equation onthe transmitted data and checks its result against the result placed inthe check sum field. If they match, no error has occurred. If they donot match, then an error occurred in the PDU.

If all of the PDUs received from redundant ATM SAR units 720 containerrors, then a re-transmission logic 750 requests that the sendingsignaling server module 412 re-transmit the particular PDU. Accordingly,Type 1 plane selection unit 730 can select a PDU on a PDU-by-PDU basis.As shown in FIG. 8, selection unit 730, replication unit 740 andre-transmission logic 750 of FIG. 8 may be implemented using a memoryand a specially programmed microprocessor 800. Here the PDU bus isreplaced with a microprocessor address/data bus. The same referencenumbers have been used in FIG. 8 to refer to the same components asthose of FIG. 7.

Type 2 selection unit 760 receives Type 2 interface data over the TDMbus. Since the AAL1 data cells do not contain a CRC code, Type 2selection unit 760 determines cell error according to a different planeselection algorithm than that above. For example, Type 2 selection unit760 may determine cell error by monitoring the signal level at ATMinterface 710 or by taking a weighted average of the selected Type 1PDUs. The AAL1 data units may also be modified to include an ATMadaption layer containing a CRC code. This would enable selection unit760 to select TDM data units in the same way Type 1 selection unit 730selects a PDU, as described above. Furthermore, since Type 2 interfaceshave little tolerance for data delays and delay variations, are-transmission logic is not associated with the Type 2 selection unit760.

Once a data cell is selected by either Type 1 plane selection unit 730or Type 2 plane selection unit 760, it is routed to either Type 1interface 780 or Type 2 interface 790. Type 1 interface 780 includesdata link level units 782 and an interface unit 784. Data link levelunits 782 receive data from plane selection unit 730, perform data linklevel functions, and output data to interface unit 784. Interface unit784 is further connected to either a signaling link, or a data network,through respective bi-directional links. Type 2 interface 790 includesinterface units 792 which receive data from plane selection unit 760 andoutput the data to a voice trunk through a bi-directional link.

When a signaling server receives data from either a signaling link, avoice trunk, or a data network, for transmission in ATM format to serverdata network 414, the data will be received at either Type 1 interface780 or Type 2 interface 790, depending upon the received data's datatype. Type 1 interface 780 forwards the received PDU data to a Type 1plane replication unit 740 which transmits replicated PDUs to ATM SARunits 720. The ATM data cells are then transmitted over each of theredundant communication paths to server data network 414. Similarly,Type 2 interface 790 forwards the received TDM data units to a Type 2plane replication unit 770 which transmits the replicated TDM data unitsto ATM SAR units 720. The ATM data cells are then transmitted over eachof the redundant communication paths to server data network 414.

FIG. 9 illustrates a second signaling server consistent with the presentinvention. The signaling server of FIG. 9 is the same as that shown inFIG. 7 with the exception that the plurality of ATM SAR units have beenreplaced with a cell steering unit 910 and a single ATM SAR unit 920.Each of the other units are the same as those shown in FIG. 7, and,therefore, will not be further described.

Cell steering unit 910 multiplexes cell data received from each ATMinterface 710 into a single cell data stream to be output to ATM SARunit 920. As shown in FIG. 10, an arbitration function unit 1010controls the multiplexing of cell steering unit 910 by controllingaccess to the cell bus by ATM interfaces 710. Arbitration function unit1010 outputs and receives controls signals from ATM interfaces 710 andSAR unit 920. When ATM interfaces 710 are ready to transmit data to SARunit 920, arbitration function unit 1010 controls bus limiters 1012 and1014 such that only one ATM interface 710 has access to the cell bus ofSAR unit 920 at any one time. At this time, arbitration function unit1010 will also control SAR unit 920 to receive the ATM data units outputover the cell bus. The plane selection algorithm then forwards the firstPDU output from SAR unit 920 having no errors, as described above inreference to FIG. 7.

In order to allow the single SAR unit 920 to differentiate which datacell belongs to which ATM interface 710, the multiplexed ATM data cellsmust be modified to permit this differentiation. As well known, each ATMcell contains a header that identifies the cell and the cell'sconnections, and a payload that follows the header in the ATM cell andcarries information intended for a recipient. The ATM header includes avirtual path identifier (VPI) and a virtual channel identifier (VCI)label, together indicating the transport connection for user informationwithin payload and other information. The VPI field of each cell canthen be modified such that it uniquely identifies the ATM interface fromwhich the particular cell originated. FIG. 11 illustrates a cellsteering unit for modifying the VPI field in the transmit direction.Processing then proceeds in the manner described above with respect toFIG. 7.

As shown in FIG. 11, when cell steering unit 910 transmits data to ATMinterface 710 for output to the data network, it duplicates each cell sothat each cell is transmitted to each ATM interface 710. Since ATM SARunit 920 outputs each cell having the same VPI field and the signalingserver transmits data units having VPI fields that identify which ATMinterface it was transmitted from, cell steering unit 910 modifies oneof the VPI fields of the data units received from ATM SAR unit 920. FIG.11 functionally illustrates this VPI modification during transmission.Cell steering unit 910 duplicates the cell received from ATM SAR unit920 and then modifies the VPI of the duplicated cell. In systemconsistent with the present invention, the VPI field prior tomodification will already be set to identify one of the ATM interfaces,and, thus, only the VPI fields of those data units pertaining to theother ATM interfaces will need to be modified. For example, as shown inFIG. 11 by functional block 1110, only the VPI field of plane 1 ismodified by cell steering unit 910.

Conclusion

Signaling servers consistent with the present invention provide auniversal, high speed, highly reliable gateway for enabling voice andsignaling communication between a data network and a PSTN. Signalingservers consistent with this invention may also be used to communicatesignaling data in place of a CCS7 network or may be used to communicatevoice data in place of a voice network, such as a typical long-distancetelephone network. These advantages are achieved through use ofredundant communication paths and error correction. It will be apparentto those skilled in the art that various modifications and variationscan be made to the system and method of the present invention withoutdeparting from the scope of the invention. The present invention coversthe modifications and variations of this invention provided they comewithin the scope of the appended claims and their equivalents.

What is claimed is:
 1. A method of communicating telephonic dataregarding a call over a data network, comprising the steps of: receivingdata units from a first data network over redundant communication paths;multiplexing the received data units of each redundant communicationpath into a single data stream; determining whether data units of themultiplexed data stream have an error; forwarding data units that do nothave errors to a second data network, wherein the receiving stepincludes the substep of receiving voice and signaling data units fromthe first data network over redundant communication paths; and thedetermining step includes determining whether received signaling dataunits have an error according to a first error check routine anddetermining whether received voice data units have an error according toa second error check routine.
 2. A method of communicating telephonicdata regarding a call over a data network, comprising the steps of:receiving data units from a first data network over redundantcommunication paths; multiplexing the received data units of eachredundant communication path into a single data stream; determiningwhether data units of the multiplexed data stream have an error;selecting one of the multiplexed units determined not to have an error;and forwarding the selected data unit to a second data network; whereineach redundant communication path terminates at an asynchronous transfermode (ATM) interface located in a signaling server module, and thereceiving step includes the substep of receiving ATM signaling dataunits, at each ATM interface, from the first data network.
 3. Asignaling server for communicating telephonic data over a data network,the signaling server comprising: a plurality of first interface unitsfor receiving signaling data units from a first data network overredundant communication paths; a cell steering unit for receiving fromthe plurality of first interface units the received data units andmultiplexing the data units into a single data stream; a segmenting andreassembling unit for receiving the multiplexed data stream; means fordetermining whether the received data units have an error; a selectionunit for selecting one of the data units determined not to have anerror; and a plurality of second interface units for forwarding theselected data unit to a second data network.
 4. A method ofcommunicating telephonic data regarding a call over a data network,comprising the steps of: receiving data units from a first data networkover redundant communication paths at a cell steering unit andmultiplexing the data units into a single data stream, including meansfor modifying the header of a received data unit to distinguish the dataunits of a redundant communication path from the data units of the otherredundant communication paths, and means for modifying a virtual pathidentifier field of asynchronous transfer mode (ATM) cells; determiningwhether data units of the multiplexed data stream have an error;selecting one of the multiplexed units determined not to have an error;forwarding the selected data unit to a second data network; receivingsignaling data units in the form of asynchronous transfer mode (ATM)cells, including a header portion, from the second data network;replicating the received ATM cells; and transmitting the received andreplicated ATM cells over the redundant communications paths to thefirst data network in a data format under which the first data networksoperates.
 5. A signaling server for communicating telephonic data over adata network, the signaling server comprising: a plurality of firstinterface units for receiving signaling data units in the form ofasynchronous transfer mode (ATM) cells, each data unit including aheader portion, from a first data network over redundant communicationpaths; a cell steering unit for receiving from the plurality of firstinterface units the received data units and multiplexing the data unitsinto a single data stream, the cell steering unit including means formodifying the header of a received data unit to distinguish the dataunits of a redundant communication path from the data units of the otherredundant communication paths and means for modifying a virtual pathidentifier field of asynchronous transfer mode (ATM) cells; means fordetermining whether the received data units have an error; a selectionunit for selecting one of the data units determined not to have anerror; a plurality of second interface units for forwarding the selecteddata unit to a second data network.
 6. A signaling server forcommunicating telephonic signaling data over data networks, thesignaling server comprising: connections to a plurality of serviceswitching points (SSPs) via signaling links; connections to one or moredata networks through redundant communications paths; a converter fortranslating telephonic signaling data units received over a signalinglink into reformatted data units for transmission over one or more datanetworks, and for translating reformatted data units received from adata network into telephonic signaling data units for transmission toone or more SSPs; an error detector for examining data units receivedover redundant communications paths and determining whether the receiveddata units contain errors; and a selection unit for selecting one of thedata units determined not to have an error for transmission.